(1) Field of the Invention
This invention relates to subtractive color filters, and more particularly to such filters including diffraction patterns for discriminating between selected wavelengths to thereby remove unwanted wavelengths of polychromatic illuminating light.
(2) Description of the Prior Art
Recently, a new technique for providing color images, particularly adapted for use in microforms such as microfilm and microfiche has been developed in which the color images are created without the use of dyes or other conventional colorants. Rather, the technique utilizes diffractive principles, by which white illuminating light is caused to be selectively dispersed depending on the color in an original colored object.
In a particularly relevant embodiment of such a technique, commonly referred to as zero order diffraction (ZOD) photography, disclosed in U.S. Pat. Nos. 3,732,363 (Glenn) and 3,957,354 (Knop), diffraction gratings are provided in which an optical image is superimposed upon the grating. The gratings are preferably first formed as a surface relief pattern in a photoresist material and are thereafter replicated in a plastic sheet to provide inexpensive copies. In U.S. Pat. No. 3,957,354, a technique for providing color subtractive filters is particularly disclosed in which the wavelength selectively attenuated by a given color filter is controlled by providing a grating having a predetermined peak optical amplitude. That patent further indicates that the periodicity (the inverse of the spatial frequency) of the gratings is important only insofar as it is sufficiently small to ensure that all diffraction orders other than the zero order are deflected beyond the aperture of the image forming optical system. Likewise, the aspect ratio, i.e., the ratio of the width of the peak to the period is disclosed to be desirably controlled, to in turn control the hue of the particular wavelength selectively attenuated. Such gratings are indicated as being formed by contact printing a chrome-on-glass diffraction grating onto a layer of photoresist having the desired thickness. As disclosed in U.S. Pat. No. 3,945,825 (Gale et al.), chrome-on-glass gratings are known to be formed by holographically exposing a photoresist layer coated on a metal film covered glass to interferring laser beams, after which the photoresist layer is developed to expose the metal film to a desired line width. The metal film is etched to form the grating in the film. The remaining photoresist is then washed off, and the plate used as a master to replicate the grating in a suitable medium. Due to the relatively slow speed of photoresists, limitations in the intensity of available coherent light sources, and the basic vibrational instability in optical element, gratings thus produced are not known to have been formed having areas greater than about 25 cm.sup.2 and typically have areas consistent with 35 mm images, i.e., an image area of about 7 cm.sup.2.
For example, in order to obtain a uniform intensity across the image plate, it is generally necessary to pass illuminating beams through apertures such that only the center of the beams are allowed to impinge on the medium. If the center portion is then spread out to provide a grating extending over a square greater than 50 cm.sup.2, the intensity is so low that the exposure time required to provide a suitable latent image on available photoresist layers becomes so long that vibrations in the optical elements cause a significant decrease in the resolution of the grating.
Accordingly, while subtractive color filters made according to the Knop patent are consistent with their use in microimaging applications, they have not heretofore been available in larger sizes, such as would be useful in overhead transparency projectors.
Another disclosure relevant to the present invention is set forth in U.S. Pat. No. 3,777,633 (Kirk). In that disclosure, optical intensity images corresponding to high-fidelity phase filters in a previously recorded and developed photoemulsion are converted to a relief image in a developed negative photopolymer layer.